An aluminium smelter may be shut-down for routine re-lining of the electrolytic cell over a period of five to eight years. During maintenance shut-down for re-lining, the maximum metal and often the bath are siphoned from a cell and the cell is allowed to cool down. The anodes are usually raised after the cell power is shut off to reduce sodium contamination. The frozen electrolyte, alumina cover, refractory lining, and cathode are subsequently dug out of the cell using tools such as jackhammers, and any of the anodes which can be recovered are removed for cleaning, a labour intensive process.
If a smelter is shut down as a result of labour conflicts, power constraints or economic downturns, it is generally desirable to shut down the electrolytic cell while keeping the cell intact unless the cell was already scheduled for re-lining. In unexpected temporary shut-downs, for example, because of electrical failure or accident, it may be possible to maintain an electrolytic cell in a “sleeping mode” with low energy applied in the cell and by lowering the anodes until they are submerged in liquid metal. Upon start-up, sufficient heat may be generated by raising the anodes close to the metal surface in order to gradually heat up the metal and sufficiently enough to pour liquid bath into the cell. Such a procedure may be dangerous, requires constant supervision and is not cost-effective if the shut-down is of prolonged duration because of the energy consumption during stoppage and the difficulty to raise the anode beam when the metal temperature is lower than 800° C.
The normal restart procedure after a complete classical stoppage requires that the electrolyte, the anodes, and alumina cover be removed in order to expose the frozen metal surface. About 20% of old anodes are generally scrapped and the good ones are cleaned and put back in the cell to be used for restart. After pre-heating the cathode to minimize the risk of explosions from moisture, molten electrolyte from so-called “donor cells” is added to cover the cathode to a depth sufficient to submerge the anodes in electrolyte. As power builds up, the anodes are raised away from the cathode to a predetermined anode-cathode distance.